Currently, waste curie monitors, being substantially an assembly of several large-area plastic scintillation detectors, are frequently used at nuclear facilities and organizations for measuring the gross gamma radioactivity of very low radioactive wastes. The aforesaid monitors are advantageous in its high radioactive sensitivity, high counting efficiency, short counting interval required for counting a test sample, no restriction to the volume of the waste to be monitored thereby, and capability of revealing the weight of the waste monitored thereby, and so on.
However, the aforesaid monitors still have drawbacks list as following: (1) The radioactivity measured thereby is often overestimated or under-estimated, since it only accounts for the weight of a waste sample tested thereby and overlooks the deviations of various self-absorption effects caused by the various densities of different waste samples. (2) Since the calibration curve used in aforesaid monitors is based upon the measurement of an object made up of a pure substance that it overlooks the fact that any usual waste sample is made up of more than one substance, the so-established calibration curve is not appropriate. (3) For calibrating the aforesaid monitor, phantom is being positioned at the geometric center of the monitor, that is not relative to the positioning of a waste sample on the floor of the aforesaid monitor as it is being measured. (4) As there is no restriction regarding to the volume of a waste sample to be measured by the aforesaid monitor, the distances between the tested sample and the scintillation detectors are not conforming to those of a phantom as being used for building a calibration curve, and thus error in radioactivity measurement will occur so that the accuracy of radioactivity measurement required by Regulation of radiation protection can not be achieved. (5) The corresponding radioactive activity with respect to the energy dependency and equation of gamma gross radioactivity for multiple radionuclides are not revised, so that the gamma gross radioactivity of the radioactive a waste sample measured thereby is not accurate.
In recent years, an improvement over conventional calibration methods of space geometric center point source is being developed for performing gross gamma radioactivity measurement upon waste samples while using the measurement for classifying wastes into radioactive wastes and non-radioactive wastes. Currently, there are several calibration methods for plastic scintillation detectors, which are listed as following: (1) The calibration method developed by Themo-Eberline adopts a means of transmission factor (TF) for calibrating self-absorption effects of various standard mass, in that, the transmission factor is defined as the ratio of the dose inside the shielding material to the outside (ambient) dose, whereas TF≦1.0 refers to the calibration of non-shield radioactive source positioned in the geometric center of a space filled with air. Moreover, as TF is defined to be 1, the mass of a radioactive material containing in a water phantom can be determined whereas 10 kg is being defined as a unit, and thus, a calibration file corresponding thereto can be established Thereby, the gross gamma radioactivity of a waste sample can be calibrated with respect to the comparison between the weight of the tested waste sample and its standard mass of TF=1; (2) The method, developed by RADOS company, Germany, adopts a pure iron plate assembly for calibrating; (3) The method, developed by Japan Nuclear Energy Safety Organization, adopts an assembly of metal tubes and metal plates for multi-radionuclides calibration; (4) The method, developed by NE Technology company, USA, adopts multi-radionuclides point sources and a pure Brazil wood of density equal to one for performing a multi-weight calibration (0˜60 kg). However, those currently available calibration methods only account for general masses and geometrical shapes, they still can not deal with the self-absorption effects of a waste sample of various masses as well as their energy dependency, and thus the gross gamma radioactivity acquired thereby is not accurate.
Therefore, it is in need of a volume calibration phantom and calibration method thereof, which free from the problems of prior arts.